We plan to study several different mechanisms which may be responsible for impulse initiation in abnormal cardiac tissues in an attempt to characterize their responses to selected pharmacological agents and patterns of electrical stimulaion. Then, using the data derived from these studies we will attempt to precisely define the mechanisms responsible for apparently similar disturbances of rhythm in the in situ heart. In parallel, we will develop means by which extracellular potential changes can be recorded to localize the initiation of arrhythmias in the in situ heart and demonstrate their mechanism in order to provide further control of our observations and conclusions. During the coming year we will emphasize arrhythmias caused by abnormal automaticity and after depolarizations.